Rough idling + knock isuses

[Slogicus]

Hi, just hoping someone has any thoughts on a rough running issue. The car is a 2015 Astra J GTC SRI with the 2.0 diesel (A 20 DTH). Everything was fine before the last service 3 weeks ago, since the service the idle from cold start is rough with fluctuations of 50-100 rpm and when it's particularly bad there is a knock from the engine almost as though the timing is being advanced.

The issue is at it's worst in hot weather but can occur at any time from a cold start. 

I've taken it back to Vauxhall who have refused to investigate properly because there aren't any fault codes. My local dealership is particularly poor when it comes to diagnosing issues so I should have expected that response but had to ask!

Any thoughts?

Many thanks!

[Rusty2009]

it is normal for the revs to fluctuate up & down abit on cold starts.

If the issues have only appeared since the work carried then they need to investigate it.